The purpose of these studies is to gain insights into the regulation of the early stages of human erythropoiesis. Although erythropoietin is essential for differentiation of late erythroid progenitors, CFU-E, into mature erythroid cells, the earliest detectable erythroid precursors, BFU-E, are not dependent on erythropoietin. Recent studies have demonstrated that murine or human mononuclear cells (lymphocytes and/or monocytes and macrophages) produce substances that enhance BFU-E proliferation and differentiation in vitro. Removal of serum or cellular sources of this material from cultures inhibits growth of BFU-E. This proposed regulator of BFU-E has been termed burst promoting activity (BPA). We will use the plasma clot culture system for human peripheral blood and marrow BFU-E. We will be using cell separation techniques to remove monocytes, macrophages, and lymphocyte subpopulations from peripheral blood and bone marrow mononuclear cells. Because we find consistent impairment of BFU-E growth when monocytes are removed from the cultured cells, we will use monocyte depleted mononuclear cells as target cells in an assay of BPA. We will examine BPA release into media conditioned by monocytes, lymphocytes, or mononuclear cells. In addition, we will study both potential stimulators of BPA production, such as mitogens and other activators of monocytes and lymphocytes, and potential inhibitors of BPA such as prostaglandins and metabolic inhibitors. We will attempt to correlate BPA production with abnormalities of erythropoiesis in vivo. We will assay BPA levels in serum and urine and BPA production by peripheral blood mononuclear cells after intensive myelosuppressive chemotherapy and in the subsequent recovery phase. We will also evaluate differences in BPA production in patients with a variety of disorders of erythropoiesis and correlate them with the proposed pathophysiology of the disease. These studies should give insight into the mechanisms of control of early erythropoietic progenitor cells, information on the physiologic relevance of BPA, and, in a broader application, some of the initial steps toward understanding how the earliest stages of stem cell proliferation and differentiation are regulated.